One conventionally known push-on switch of this type is a dual-stage switch in which a first-stage switch turns on when the switch is lightly pressed, and then a second-stage switch turns on when the switch is more firmly pressed. Such a push-on switch is frequently used in an input operation section, such as a shutter release button of a camera or a record start button of a video camcorder.
FIG. 11 is an exterior perspective view of a conventional push-on switch. FIG. 12 is an exploded perspective view of the push-on switch of FIG. 11. FIG. 13A is a cross-sectional view taken along the line P-P shown in FIG. 11. FIG. 13B is a cross-sectional view taken along the line Q-Q shown in FIG. 11.
FIGS. 11 to 13B show substantially rectangular switch enclosure 1 that is made of resin and includes substantially circular recess 1A with an open upper side. On the inner bottom of substantially circular recess 1A, as shown in FIG. 14, which is described later, there are provided two electrically isolated peripheral contacts 2 and center contact 3 that has a slightly lower height than peripheral contacts 2. Those bare, fixed contacts are formed by insert molding.
Connection terminals 2A, 2B and 3A electrically continuous with respective fixed contacts are led out to outer sides of switch enclosure 1 and protrude therefrom. Connection terminal 6A, which is a dummy terminal, is also provided on an outer side of switch enclosure 1 and protrudes therefrom.
There are provided a pair of step-shaped movable contact receptacles 4 at opposite positions on side walls that form recess 1A. There are also provided a pair of oppositely positioned cutout grooves 5 along a line perpendicular to a center line connecting the two movable contact receptacles 4.
As shown in the plan and side views of FIGS. 15A and 15B, movable contact 7, which is formed by press working elastic sheet metal, includes spherically, upwardly curved center disc segment 8; circular ring segment 10 that is concentrically disposed outside disc segment 8 with a fixed distance therebetween; a pair of inclined connecting segments 9 that are disposed at symmetrical positions with respect to the center of movable contact 7 and that connect disc segment 8 and ring segment 10 such that disc segment 8 is higher than ring segment 10; and a pair of linearly projecting segments 11 that protrude outward from ring segment 10. Ring segment 10 is curved upward in a substantially V-shape such that two opposite positions disposed perpendicular to connecting segments 9 are two apexes 10B.
FIG. 16 is a plan view showing movable contact 7 mounted in switch enclosure 1. Projecting segments 11 are inserted in cutout grooves 5 such that the convexly curved side of movable contact 7 is oriented to project upwardly. Side sections 10A of the V-shaped bends of ring segment 10 are disposed in recess 1A of switch enclosure 1 such that they are mounted on movable contact receptacles 4, as shown in FIG. 13B.
The upper side of switch enclosure 1 is covered with flexible insulating film sheet 12, as shown in FIGS. 13A, 13B and 12. Metal cover 13 is attached to switch enclosure 1 via sheet 12. Sheet 12 is sandwiched between the lower side of metal cover 13 and the upper end face of switch enclosure 1 as well as low-profile projection 1B disposed on the upper end face of switch enclosure 1 such that projection 1B surrounds recess 1A.
As shown in FIG. 11 and other figures, metal cover 13 includes a pair of elastic arms 13B extending from flat frame-like segment 13A toward the center of metal cover 13, and center pressing segment 13C joined with flat frame-like segment 13A via elastic arms 13B. Pressing segment 13C has a substantially circular periphery and is provided with downward projection 13D at its center that protrudes downward.
The thus configured conventional push-on switch is normally of f because movable contact 7 is apart from peripheral contacts 2 and center contact 3, as shown in FIG. 13A.
The operation of the above push-on switch when it is pressed will be described with reference to cross-sectional views of FIGS. 17A, 17B, 18A and 18B. FIGS. 17A and 18A are cross-sectional views taken along the line P-P shown in FIG. 11. FIGS. 17B and 18B are cross-sectional views taken along the line Q-Q shown in FIG. 11. For clarity, these drawings only show main segments for metal cover 13.
When pressing segment 13C of metal cover 13 is lightly pushed from above, pressing segment 13C moves downward and downward projection 13D presses disc segment 8 of movable contact 7 via sheet 12. When the pressing force exceeds a predetermined value, as shown in FIGS. 17A and 17B, ring segment 10 of movable contact 7, supported by the portions of ring segment 10 placed on movable contact receptacles 4, provides a moderate click feeling as it changes from the convex shape to a concave shape while disc segment 8 keeps its spherical shape. At this point, disc segment-side base parts of two connecting segments 9 come into contact with two electrically isolated peripheral contacts 2, respectively. This first-stage action turns the first-stage switch on where two peripheral contacts 2, that is, connection terminals 2A and 2B (see FIG. 16) become electrically continuous with each other via movable contact 7.
When pressing segment 13C is further pushed downward from this state to push disc segment 8 of movable contact 7 downward via sheet 12, disc segment 8, which is now supported by its periphery placed on peripheral contacts 2, provides a moderate click feeling as it changes from the convex shape to a downward concave shape, as shown in FIGS. 18A and 18B, and the center lower side of disc segment 8 comes into contact with center contact 3. This second-stage action turns the second-stage switch on where center contact 3 as well as two peripheral contacts 2, which were already short-circuited when the first-stage switch was turned on, that is, connection terminal 3A as well as connection terminals 2A and 2B (see FIG. 16) become electrically continuous with each other.
Thereafter, when the pushing force on pressing segment 13C is removed, the pressing force on disc segment 8 of movable contact 7 is removed and disc segment 8 first restores its upward protruding spherical shape due to its elastic restoring force. Consequently, the center lower side of disc segment 8 separates from center contact 3 and the second-stage switch returns to its off-state, followed by the movement of connecting segments 9 returning to their inclined state in which their disc segment 8 side sections become higher than the other side. At the same time, disc segment-side parts of the connecting segments 9 separate from peripheral contacts 2 and the first-stage switch also returns to its off-state. When movable contact 7 is restored to its original shape, sheet 12 as well as elastic arms 13B joined with pressing segment 13C return to their original positions.
FIG. 19 shows the relationship between the travel in each operation described above and the timing of when the switch at each stage turns on.
Such a conventional push-on switch is equipped as a switch of a shutter release section of a digital still camera, for example, and an on-signal supplied from the first-stage switch by a light press operation activates focus adjustment for a subject. Another on-signal supplied from the second-stage switch by a firmer press operation fires the shutter.
A known related art document associated with the invention of this application is, for example, Japanese Patent Unexamined Publication No. 1999-232962.
The above-mentioned conventional push-on switch is equipped, for example, in shutter release sections of various cameras. On the other hand, as digital still cameras and video camcorders become commonplace, such cameras themselves have been modified in various ways and provided with enhanced functionality and user friendly features.
Under the current situation in which cameras with an anti-handshake function are especially well accepted in the market, when the above-mentioned conventional push-on switch is used as a shutter release button or a record start button of such a camera with an anti-handshake function, the on-signal from the first-stage switch activates the anti-handshake function as well as focus adjustment. To keep these functions activated, after the first moderate click feeling of the switch is provided, this pressed state must be retained. In this case, a relatively low operation force is sufficient to keep the first-stage switch on. However, when taking pictures while the user is moving, problems arise. For example, the finger may slightly come off the operation button or the above activated state may undesirably be released. Therefore, there is a desire to achieve a push-on switch that overcomes such problems.